The embodiments described herein relate generally to a variable speed machine and, more particularly, to a variable speed machine assembly for use with a fluid turbine, such as a wind turbine.
At least some known wind turbines include machines for converting variable speed mechanical input from blades of the wind turbine into electric power that is compliant with an electrical grid. For example, some known wind turbines include a doubly fed induction generator (DFIG) for converting the variable speed mechanical input. The DFIG includes a converter that has a rating that is a percentage of a rated capacity of the wind turbine, such as about 35% of rated capacity. Such a converter is also referred to as a partial-rated converter. Further, the DFIG includes slip rings and may have less reactive power control and/or lower efficiency, as compared to other variable speed machines.
At least some other known wind turbines include permanent magnet generators (PMG) that have a higher efficiency than at least some known DFIGs. PMGs include a converter at the rated capacity, also referred to as a full-rated converter. Typically, full-rated converters include complex power electronics, which have disadvantages, such as large size, high maintenance requirements, and/or high cost, as compared to partial-rated converters.